Abstract: I evaluated the breakdown of yellow birch leaves in the presence and absence of P. gentilis using a leaf pack breakdown experiment. Leaf packs were placed in reaches above the dam where P. gentilis is present and below where it is essentially absent. I also conducted experiments to determine why P. gentilis was uncommon below the dam. Using enclosures, I evaluated whether there were differences in the survival and growth of P. gentilis in the two stream sections. I also evaluated the potential for intraguild predation by a larger caddisfly, Pycnopsyche luculenta, which is more abundant below the dam. Pycnopsyche gentilis larvae were placed in enclosures with or without P. luculenta above and below the dam; all enclosures included leaf packs. Leaf breakdown was significantly slower in the downstream section. Pycnopsyche gentilis was absent from downstream leaf packs. Detritivore diversity indices were similar for both stream sections. Calculations of P. gentilis per capita leaf ingestion in enclosures suggest that P. gentilis was primarily responsible for the higher breakdown rates in the upstream reach. My results are consistent with those of other studies that suggest the presence of P. gentilis and not diversity drives detrital breakdown in southern Appalachian streams.